The present invention relates to an automatically operated freezing plant comprising a plate freezer having a stack of horizontal freezing plates and stepping means for subjecting the stack to a succession of lifting steps each followed by a smaller lowering step in such a manner as to successively place each plate except the uppermost one at a charging and discharging level. For each step, the respective upwardly next plate is detained at a distance above the charging and discharging level sufficient to permit the charging of freezing goods from one side of the stack into the enlarged interspace between the two plates defining the charging and discharging level. At the same time, simultaneous discharging of frozen goods from the interspace at the other side of the stack occurs. Means are also provided for lowering the whole stack to its original position after freezing and after frozen goods have been charged into and discharged from the interspace between the lowermost two plates. The plate freezer is combined with a plurality of identical plate freezers to form a series of plate freezers in horizontally aligned arrangement with the charging sides and discharging sides of the stacks situated in common planes.
It is known to use a stepping arrangement as above described in so-called manually operated horizontal plate freezers. In the operation of these, upon lowering of the stack of plates to its original position, a pressure is applied to the stack, whereafter a freezing period is initiated during which the product is frozen followed by a thawing loose period, whereafter the emptying and filling operations are repeated. Such manually operated plate freezer has the advantage that since pressure is applied to the product to be frozen during the freezing period, a compact structure of the frozen material is achieved and, accordingly, air pockets and uneven surfaces are avoided. However, such plate freezer suffers from the drawback that the feeding of the product to be frozen to the plate freezer and the removal of the product must be carried out intermittently because the feeding of the product to and the removal of the frozen product from the plate freezer must be stopped during the freezing period and the thawing loose period.
So-called automatic plate freezers of the kind referred to which allow more or less continuous feeding of the product to be frozen to the freezer and more or less continuous removal of the product are disclosed in GB 2,145,805, U.S. Pat. No. 4,240,470 and U.S. Pat. No. 4,553,406. In these known freezers, cooling medium is fed to the plates all the time, and when an emptying/filling cycle has been terminated the whole stack is lowered, whereafter the emptying/filling cycle is repeated immediately or with a minimum of delay. During the freezing the product is not subjected to other pressure forces than the pressure forces which the plates apply to each other. The plate, which at any time forms "ceiling" above the emptying/freezing interspace will be subjected to the pressure from the plates positioned there above, and the plate which at any time is positioned at the bottom of the stack will be subjected to the pressure from the plates arranged there above inclusive the plate which forms "floor" of the emptying/filling interspace. Accordingly, the product positioned below the uppermost plate will at the maximum be subjected to the pressure from this plate and parts which may be connected thereto, if any. Furthermore, since the plate freezer does not have any thawing loose period it may be necessary to use violence upon the frozen product when this is to be removed.
It is the object of the present invention to provide a freezing plant which may operate continuously without the drawbacks connected with the above mentioned automatic plate freezer, and which, accordingly, results in the advantages which are achieved by means of the manually operated plate freezer, and this object is according to the invention achieved by constructing the freezing plant according to the present invention described above. Hereby it is achieved that the freezing plant in question can operate continuously because one plate freezer may be emptied and filled step by step while the other freezers freeze the product in question and provide for thawing the product loose, but in such a way that the next plate freezer is available for commencing its first emptying and filling step immediately after the beforegoing plate freezer has completed its last emptying and filling step. Accordingly, it is achieved that the series may be fed with the product to be frozen in a continuous way and the product may also continuously be removed from the series and an excellent product is achieved because pressure is applied to the product during the freezing period, and due to the thawing loose period the withdrawal of the frozen product causes no difficulties.
A freezing plant consisting of a series of vertical plate freezers is known from Prof. Dipl. Ing. Heinrich Drees, "Kuhlanlagen", published 1972 by VEB Verlag Technik Berlin, see pages 273-274. In this known freezing plant the individual vertical plate freezers are in turn subjected to a thawing medium and emptied and then re-filled in a bath-wise operation under the control of the operator. The known freezing plant therefore does not allow for the establishment of an uninterrupted smooth flow of freezing and frozen goods through the freezing plant from an input conveyor to an output conveyor without human attendance.